Credit: Stuart Cahill

Credit: Christopher Evans

A member of the Boston Medical Center staff runs near the emergency room during the response to today's Boston Marathon bombing.

Credit: Christopher Evans

An injured person is transported to Tufts Medical Center after the explosions at the Boston marathon finish line.

Credit: John Wilcox

Bridget Peterson and her daughter Laura Peterson leave the MGH where they they met after witnessing the explosions at the Boston Marathon.

Credit: Matt Stone

Nicole Niles of Manchester NH who ran in the Marathon hugs Kerry Smith of Wakefield after the Marathon bombing.

Credit: Matthew Healy

Workers from Tufts Medical Center wait across the street from the hospital after a bomb scare inside.

Credit: Matt Stone

Metro SWAT guard the Taj Hotel after the Marathon bombing.

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Boston is in mourning today for three killed and at least 144 wounded in a “cowardly” bombing at the Boston Marathon, leaving the tragedy-wracked city scrambling for answers as to how a terrorist was able to plant two explosive devices on a busy stretch of Boylston Street at a high-security international event.

“This cowardly act will not be taken in stride,” Boston Police Commissioner Edward F. Davis said. “We will turn over every rock to find those responsible.”

The 2:50 p.m. double bombing shattered the bliss of the sun-splashed marathon, sending frantic runners and fans scrambling for cover and sending at least 144 to the hospital. There were at least three dead — including an 8-year-old — and 30 wounded “critically,” officials said last night.

The FBI took over the investigation last night as President Obama pledged to track down the ruthless killer or killers.

“We will get to the bottom of this. We will find out who did this and we will find out why,” President Obama said. “They will feel the full weight of justice.

“Boston is a tough and resilient town. Residents will pull together, take care of each other and move forward,” he added. “The American people are with them every step of the way.”

FBI Special Agent-in-Charge Rick DesLauriers said the city should remain in a “heightened state of vigilance” today as an army of cops patrols the streets.

“It is a potential terrorist investigation,” DesLauriers said last night. “We will bring those responsible to justice as quickly as possible.”

Late last night, state police, Boston bomb squad and canine units, as well as FBI, ICE and ATF agents, swooped into an Ocean Avenue apartment complex in Revere. It was unclear whether the search was connected to the marathon bombings.

Downtown, the city was gripped by fear as revelers who came to celebrate wound up running for their lives. Roughly 15 blocks around Copley Square were “locked down” after the attacks and were slated to remain so today. Heavily armed National Guard soldiers, state troopers with bomb-sniffing dogs and hundreds of cops patrolled hotels, restaurants and the streets while shaken fam­i­- lies huddled inside.

Officials said random bag searches would occur today on MBTA trains.

There were reports last night of a suspect in custody, but Davis said they were wrong. He did say there were “people being questioned,” but provided no more details.

Mayor Thomas M. Men­ino, who was hospitalized with a broken leg, checked himself out of Brigham and Women’s Hospital last night to be at the city’s command post, where he met with Davis, Gov. Deval Patrick, and federal and state officials.

“The marathon is a great day in the city of Boston, but we had a tragedy,” Men­ino said from a wheelchair at the Westin Copley hotel. “I offer my condolences and prayers to the victims.”

Logan International Airport ­— the launching pad for two of the planes in the Sept. 11, 2001, attacks in New York City — was shut down for about an hour after the attack. A no-fly zone around Boston was put in place by the Federal Aviation Administration.

In a statement, the Boston Athletic Association, which organizes the marathon, said: “It is a sad day for the City of Boston. ... What was intended to be a day of joy and celebration quickly became a day in which running a marathon was of little importance.”

Kyle Loehning of Charlestown arrived at Tufts Medical Center to see his sister-in-law, who was injured in the blast.

“She was in the wrong place at the wrong time,” he said. “They think she lost her leg.”

U.S. Rep. Stephen F. Lynch, a South Boston resident, said authorities believe the devices were brought into the area after police did a sec­urity sweep. He said city and state officials took “all the appropriate steps,” but sec­- urity efforts in the future “will have to be redoubled.”

“There was nothing to lead us to believe that what we were doing, what the mayor was doing, what the governor was doing was less than appropriate,” Lynch said. “The staffing was there, the sweeps were there. ... They did their due diligence. But ... under these circumstances, it was very difficult to stop.”